Polycrystalline silicon rod, processing method for polycrystalline silicon rod, method for evaluating polycrystalline silicon rod, and method for producing FZ single crystal silicon

ABSTRACT

For evaluating a polycrystalline silicon rod to be used as a raw material for production of FZ Si single crystals, novel evaluation values (values of characteristics×amount of crystals) including the amount of crystals grown in the growth direction (radial direction) are defined and the homogeneity in crystal characteristics in the growth direction (radial direction) is evaluated. Specifically, the homogeneity of the polycrystalline rod is evaluated by sampling a plurality of specimen plates each having, as a principal plane thereof, a cross-section perpendicular to a radial direction of the polycrystalline rod grown by a Siemens method at equal intervals in the radial direction, determining values of characteristics of the crystals of the specimen plates by measurements, and by using evaluation values obtained by multiplying amounts of the crystals (relative amounts of the crystals) at sites where the specimen plates have been sampled by the values of the crystal characteristics.

TECHNICAL FIELD

The present invention relates to a technology for producing apolycrystalline silicon rod, more particularly to a technology forproducing a polycrystalline silicon rod suitable as a raw material forproduction of single crystal silicon by an FZ method.

BACKGROUND ART

Crystals of single crystal silicon essential for production ofsemiconductor devices and the like are grown by a CZ method or an FZmethod, and polycrystalline silicon ingots and polycrystalline siliconrods are used as raw materials at this time. Such polycrystallinesilicon materials are produced by a Siemens method in many cases (seePatent Literature 1 (Japanese Patent Publication No. 37-18861) and thelike). The Siemens method refers to a method in which a silane rawmaterial gas such as trichlorosilane or monosilane is brought intocontact with a heated silicon core wire to thereby vapor-phase grow(deposit) polycrystalline silicon on the surface of the silicon corewire by a CVD (Chemical Vapor Deposition) method.

For example, when a crystal of single crystal silicon is grown by the CZmethod, a polycrystalline silicon ingot is charged in a quartz crucible;a seed crystal is dipped in a silicon melt made by heating and meltingthe ingot and dissipating dislocation lines; while the seed crystal is,after being non-dislocated, gradually enlarged in diameter until apredetermined diameter is attained, the crystal is pulled upwards at aconstant rate. At this time, if unmelted polycrystalline silicon remainsin the silicon melt, the unmelted polycrystalline pieces drift in thevicinity of the solid-liquid interface by convection, causing inducinggeneration of dislocations and losing crystal lines.

Patent Literature 2 (Japanese Patent Laid-Open No. 2014-001096)discloses a method for evaluating the degree of crystalline orientationof polycrystalline silicon by an X-ray diffractometry, with the purposeof providing a technology of selecting in high quantitativity andrepeatability the polycrystalline silicon suitable as a raw material forproduction of single crystal silicon, and contributing to the stableproduction of the single crystal silicon.

Patent Literature 2 proposes, in consideration of the problem that evenif the polycrystalline silicon is one whose crystal grains cannot berecognized by the conventional visual observation, when single crystalsilicon is produced by using the polycrystalline silicon as a rawmaterial, loss of crystal lines is caused by being induced by generationof dislocations in some cases, as novel means, a method for evaluatingthe degree of crystalline orientation of polycrystalline silicon by anX-ray diffractometry, in which method a sampled specimen disc isarranged on a position where the Bragg reflection from a Miller indexplane <hkl> is detected; the specimen disc is rotated in-plane in arotational angle of ϕ with the center of the specimen disc as therotational center so that an X-ray radiation region established by aslit ϕ-scans the principal plane of the specimen disc; a chart isacquired, which indicates the dependency of the Bragg reflectionintensity from the Miller index plane <hkl> on the rotational angle (ϕ)of the specimen disc; a baseline is determined from the chart; and thevalues of the diffraction intensity of the baseline is used as anevaluation index of the degree of crystalline orientation.

The Patent Literature then construes that when single crystal silicon isgrown by using a polycrystalline silicon rod or a polycrystallinesilicon ingot selected based on the evaluation result of the abovemeans, the generation of loss of crystal lines can be prevented in ahigh probability.

It is also reported that a polycrystalline silicon rod, in which,particularly as seen in the above-mentioned silicon rods A and B, avalue obtained by dividing a maximum value of a plurality of values ofbaseline diffraction intensity for the Miller index plane <111> by aminimum value thereof is 1.5 or lower; also a value obtained by dividinga maximum value of a plurality of values of baseline diffractionintensity for the Miller index plane <220> by a minimum value thereof is1.9 or lower; and the division value (I<111>/I<220>) is lower than 2.5for either specimen plate, is suitable as a raw material for growth ofsingle crystal silicon.

Patent Literature 3 (Japanese Patent Laid-Open No. 2013-217653) alsodiscloses a method for evaluating the degree of crystalline orientationof polycrystalline silicon by an X-ray diffractometry, with the purposeof providing a technology of selecting in high quantitativity andrepeatability the polycrystalline silicon suitable as a raw material forproduction of single crystal silicon, and contributing to the stableproduction of the single crystal silicon.

Specifically, it is disclosed that when a specimen disc sampled from apolycrystalline silicon rod is evaluated, peaks may emerge on a ϕ-scanchart in some cases and the smaller the number of the peaks and thenarrower the half-value widths of the peaks, the more suitable thepolycrystalline silicon rod as a raw material for production of singlecrystal silicon, and it is reported that the number of the peaksemerging on the ϕ-scan chart is preferably 24 peaks/cm² or less per unitarea of the specimen disc for either of the Miller index planes <111>and <220>; and that it is preferable to select a polycrystalline siliconrod having an inhomogeneous crystal grain diameter of smaller than 0.5mm for either thereof as a raw material for production of the singlecrystal silicon, where the inhomogeneous crystal grain diameter isdefined as a value obtained by multiplying a half-value width of a peakby δL=2^(1/2)πR₀/360 obtained, wherein R₀ is the radius of the specimendisc.

Patent Literature 4 (Japanese Patent Laid-Open No. 2014-031297) alsodiscloses a method for selecting a polycrystalline silicon rod to beused as a raw material for production of single crystal silicon, withthe purpose of providing a technology of selecting in highquantitativity and repeatability the polycrystalline silicon suitable asa raw material for production of the single crystal silicon, andcontributing to the stable production of the single crystal silicon.

Specifically, an electron backscatter diffraction image acquired byirradiating the principal plane of a specimen plate sampled from apolycrystalline silicon rod with electron beams is analyzed; and, as araw material for production of single crystal silicon, selected is apolycrystalline silicon rod simultaneously satisfying (condition 1) thatthe sum total area of regions where no crystal grains of 0.5 μm orlarger in grain diameter are detected is 10% or smaller of the wholearea irradiated with the electron beams, and (condition 2) that thenumber of crystal grains in the range of 0.5 μm or larger and smallerthan 3 μm in grain diameter is 45% or more of the total number ofcrystal grains detected. The Patent Literature reports that when singlecrystal silicon is grown by using such a polycrystalline silicon rod,since no loss of crystal lines is caused, the stable production of thesingle crystal silicon becomes possible.

Patent Literature 5 (Japanese Patent Laid-Open No. 2014-034506) alsodiscloses a method for selecting a polycrystalline silicon rod to beused as a raw material for production of single crystal silicon, withthe purpose of providing a technology of selecting in highquantitativity and repeatability the polycrystalline silicon suitable asa raw material for production of the single crystal silicon, andcontributing to the stable production of the single crystal silicon.

Specifically, the method is a method in which a specimen plate with across-section as the principal surface perpendicular to the radialdirection of a polycrystalline silicon rod grown by deposition by achemical deposition method is sampled; the thermal diffusivity α(T) ofthe specimen plate is measured; and a polycrystalline silicon rodsuitable as a raw material for production of single crystal silicon isselected based on a ratio (α(T)/α_(R)(T)) of thermal diffusivities incomparison with the thermal diffusivity, α_(R)(T), of a standardspecimen. The Patent Literature reports that when single crystal siliconis grown by using a polycrystalline silicon rod thus selected, no lossof crystal lines occurs and thus stable production of the single crystalsilicon becomes possible.

Any of the methods disclosed in these Patent Literatures evaluatesvalues of characteristics and distributions thereof in the growthdirection (diameter direction) of polycrystalline silicon rods, but anythereof does not put, as evaluation subjects, the rotational symmetry inthe growth direction (radial direction) in the polycrystalline siliconrods, the homogeneity of crystals in the extension direction (axialdirection) thereof, and the like.

In polycrystalline silicon rods to be used as a raw material forproduction of single crystal silicon by an FZ method, similarly to thedistribution states of various characteristics in the growth direction(radial direction), the distribution states of various characteristicsin the extension direction (axial direction) are important factors.Here, characteristics of crystals refer to, for example, the crystalformation amount, the crystal orientation, the crystal grain diameter,the thermal diffusivity and the thermal conductivity.

This is because when single crystal silicon is produced by an FZ method(floating zone melting method), a polycrystalline silicon rod iszone-melted while being rotated, but the homogeneity and the like ofcrystal characteristics in the extension direction (axial direction)have not been put so far as evaluation subjects.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Publication No. 37-18861

Patent Literature 2: Japanese Patent Laid-Open No. 2014-001096

Patent Literature 3: Japanese Patent Laid-Open No. 2013-217653

Patent Literature 4: Japanese Patent Laid-Open No. 2014-031297

Patent Literature 5: Japanese Patent Laid-Open No. 2014-034506

SUMMARY OF INVENTION Problem to be Solved

In order to obtain a polycrystalline silicon rod suitable as a rawmaterial for production of single crystal silicon by an FZ method,however, it is needed to enhance “homogeneity in crystalcharacteristics” of the polycrystalline silicon rod as a whole, that is,not only the homogeneity in the growth direction (radial direction) inthe polycrystalline silicon rod, but also the homogeneity in theextension direction (axial direction).

Additionally, not only the “homogeneity in crystal characteristics” butalso the “homogeneity in shape” of the polycrystalline silicon rod isimportant. This is because when the “homogeneity in shape” in theextension direction (axial direction) of the polycrystalline silicon rodis low, since heat does not transfer uniformly, crystal habit linesindicating being in the non-dislocation state are lost (dislocated) insome cases.

The present invention has been achieved in consideration of suchsituations, and has an object to provide a technology for providing apolycrystalline silicon rod, excellent in the homogeneity in shape andthe homogeneity in crystal characteristics, suitable as a raw materialfor production of single crystal silicon by an FZ method.

Solution to Problem

In order to solve the above problem, a polycrystalline silicon rod of afirst aspect according to the present invention is a polycrystallinesilicon rod to be used as a raw material for production of singlecrystal silicon by an FZ method, wherein the polycrystalline silicon rodhas an entire length of 500 mm or larger and a difference ΔD₁ between amaximum value D_(max) and a minimum value D_(min) of diameters over theentire length is 3 mm or smaller.

For example, the average diameter of the polycrystalline silicon rod is300 mm or smaller.

A polycrystalline silicon rod of a second aspect according to thepresent invention is a polycrystalline silicon rod to be used as a rawmaterial for production of single crystal silicon by an FZ method,wherein the polycrystalline silicon rod has an entire length of 500 mmor larger, and a difference ΔD₂ of 6 mm or smaller between a maximumvalue D_(max) and a minimum value D_(min) of diameters of a nearlycylindrical virtual rotating body formed at a time when thepolycrystalline silicon rod is rotated about the center axis in theextension direction thereof.

For example, the average diameter of the polycrystalline silicon rod is300 mm or smaller.

A method for processing a polycrystalline silicon rod of the firstaspect according to the present invention is a method for processing apolycrystalline silicon rod to be used as a raw material for productionof single crystal silicon by an FZ method, wherein the polycrystallinesilicon rod is a polycrystalline silicon rod having an entire lengthgrown by a Siemens method of 500 mm or larger; and a side surface of thepolycrystalline silicon rod is cylindrically ground so that thedifference ΔD₁ between a maximum value D_(max) and a minimum valueD_(min) of diameters over the entire length becomes 3 mm or smaller.

A method for processing a polycrystalline silicon rod of the secondaspect according to the present invention is a method for processing apolycrystalline silicon rod to be used as a raw material for productionof single crystal silicon by an FZ method, wherein the polycrystallinesilicon rod is a polycrystalline silicon rod by a Siemens method havingan entire length grown of 500 mm or larger; and a side surface of thepolycrystalline silicon rod is cylindrically ground so that a differenceΔD₂ between a maximum value D_(max) and a minimum value D_(min) ofdiameters of a nearly cylindrical virtual rotating body formed when thepolycrystalline silicon rod is rotated about the center axis in theextension direction thereof becomes 6 mm or smaller.

A method for evaluating crystals of a polycrystalline silicon rodaccording to the present invention is a method for evaluating crystalsof a polycrystalline silicon rod to be used as a raw material forproduction of single crystal silicon by an FZ method, wherein aplurality of specimen plates each having, as the principal planethereof, a cross-section perpendicular to a radial direction of thepolycrystalline silicon rod grown by a Siemens method are sampled atequal intervals in the radial direction; values of a crystalcharacteristic of the specimen plates are determined by measurements;and the homogeneity of the polycrystalline silicon rod is evaluated byusing evaluation values obtained by multiplying relative amounts of thecrystals at sites where the specimen plates have been sampled by thevalues of the crystal characteristics.

In an aspect, the method comprises a step of determining the aboveevaluation values of two specimens sampled from sites at symmetricalpositions about the center axis of the polycrystalline silicon rod,calculating a rotational symmetry from the obtained two evaluationvalues A and B by the following expression:100−(|A−B|/(A+B)/2)×100,and evaluating whether or not the average value in the axial directionof the rotational symmetries is 40% or higher.

Further, in an aspect, the method comprises a step of determining theabove evaluation values of two specimens sampled from sites positionedat equal distances in the radial direction from the center axis of thepolycrystalline silicon rod and on the same generating line in the casewhere the polycrystalline silicon rod is approximated to a cylinder,calculating a difference in rotational symmetry from the obtained twoevaluation values C and D by the following expression:(|C−D|/(C+D)/2)×100,and evaluating whether or not the average value in the axial directionof the differences in rotational symmetry is 40% or lower.

In the above-mentioned method for evaluating crystals of apolycrystalline silicon rod, the crystal characteristic is, for example,any of an amount of crystals famed, a crystal orientation, a crystalgrain diameter, a thermal diffusivity and a thermal conductivity.

The method for producing FZ single crystal silicon according to thepresent invention is a method for producing FZ single crystal silicon byusing a polycrystalline silicon rod as a raw material, wherein thepolycrystalline silicon rod is a polycrystalline silicon rod collectedfrom a leg section of a reverse U-shape polycrystalline silicon sectionobtained by assembling one pair (two wires) of silicon core wiresthrough a bridge in a reverse U-shape and depositing polycrystallinesilicon; and when one end side of the polycrystalline silicon rod is aportion thereof in the vicinity of the bridge in a deposition furnace,and the other end side thereof is a portion thereof in the vicinity ofan electrode in the deposition furnace, a floating zone melting step isinitiated from the portion in the vicinity of the bridge of thepolycrystalline silicon rod.

Advantageous Effects of Invention

In the present invention, novel evaluation values are introduced, whichare “values of characteristics×relative amount of crystals”, and apolycrystalline silicon rod having a “rotational symmetry” of 40% orhigher based on the evaluation values is used as a polycrystallinesilicon rod suitable as a raw material for production of single crystalsilicon by an FZ method. Further, a polycrystalline silicon rod having a“difference in rotational symmetry” of 40% or lower is used as apolycrystalline silicon rod suitable as a raw material for production ofsingle crystal silicon by an FZ method.

According to the present invention, there is provided a technology forproviding a polycrystalline silicon rod, excellent in the homogeneity inshape and the homogeneity in crystal characteristics, suitable as a rawmaterial for production of single crystal silicon by an FZ method.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view for interpreting one example of a method of samplingspecimens used for evaluation of values of characteristics.

FIG. 2 is a graphic diagram of evaluation values (I^(<111>)×Δd² andI^(<220>)×Δd²) obtained from 32 sheets in total of 16 sheets of specimenplates sampled from a portion in the vicinity of the bridge and 16sheets thereof sampled from a portion in the vicinity of the electrode.

DESCRIPTION OF EMBODIMENTS

As a result of exhaustive studies in consideration that it is neededthat a polycrystalline silicon rod suitable as a raw material forproduction of single crystal silicon by an FZ method be high in the“homogeneity in shape” in the extension direction (axial direction) ofthe polycrystalline silicon rod, and further, be high not only in thehomogeneity in crystal characteristics in the growth direction (radialdirection) of the polycrystalline silicon rod but also in thehomogeneity in crystal characteristics in the extension direction (axialdirection) thereof, the present inventors have achieved the presentinvention. Hereinafter, embodiments to carry out the present inventionwill be described by reference to the drawings.

[Homogeneity in Shape]

The cross-section of a polycrystalline silicon rod right after beingobtained by a Siemens method is not a complete true circle but a slightellipse in many cases. Additionally, the cross-sectional shape acrossthe extension direction (axial direction) depends on sites. That is, apolycrystalline silicon rod as it is grown by a Siemens method is not“homogeneous in shape” in the extension direction (axial direction).

According to studies by the present inventors, when the “homogeneity inshape” in the extension direction (axial direction) of a polycrystallinesilicon rod is low, in the case where the polycrystalline silicon rod isused as a raw material for production of single crystal silicon by an FZmethod, crystal habit lines indicating being in the non-dislocationstate are lost (dislocated) in some cases. This is because the constantheat conduction is not carried out in the successive state change ofmelting, convection and solidification by high-frequency heating.

Then, studies have been made on what degree of “circularity” of thecross-section of a polycrystalline silicon rod is suitable for a rawmaterial for production of single crystal silicon by an FZ method, andfurther on what degree of the difference between a maximum value and aminimum value of diameters of a nearly cylindrical virtual rotating bodyformed when the polycrystalline silicon rod is rotated about the centeraxis in the extension direction is suitable for a raw material forproduction of single crystal silicon by an FZ method. Here, theevaluation was carried out using a polycrystalline silicon rod having anentire length of 500 mm or larger in entire length and an averagediameter of 300 mm or smaller (5 to 12 inches in diameter), and thejudgment of “being suitable” was made by the presence/absence of loss ofcrystal habit lines.

As a result, the finding has been acquired that when single crystalsilicon is produced by using a polycrystalline silicon rod satisfyingthe following conditions by an FZ method, no loss of crystal habit linesis recognized.

That is, with respect to the “circularity” of cross-sections of thepolycrystalline silicon rod, the condition is such that the differenceΔD₁, between a maximum value D_(max) and a minimum value D_(min) ofdiameters over the entire length, is 3 mm or smaller.

Further with respect to the difference between a maximum value and aminimum value of diameters of a nearly cylindrical virtual rotating bodyformed when a polycrystalline silicon rod is rotated about the centeraxis in the extension direction, the condition is such that thedifference ΔD₂ between the maximum value D_(max) and the minimum valueD_(min) is 6 mm or smaller. Here, the difference ΔD₂ between the maximumvalue D_(max) and the minimum value D_(min) corresponds to a variationwidth in the radial direction when a polycrystalline silicon rod isclasped and rotated in an actual FZ process.

That is, the polycrystalline silicon rod according to the presentinvention is a polycrystalline silicon rod to be used as a raw materialfor production of single crystal silicon by an FZ method, wherein thepolycrystalline silicon rod has an entire length of 500 mm or larger,and a difference ΔD₁ between a maximum value D_(max) and a minimum valueD_(min) of diameters over the entire length is 3 mm or smaller.

In the case where the shape of a polycrystalline silicon rod right afterbeing obtained by a Siemens method does not satisfy the aboveconditions, the polycrystalline silicon rod comes to be formed bycylindrically grinding the side surface of the polycrystalline siliconrod.

That is, the method for processing a polycrystalline silicon rodaccording to the present invention is a method for processing apolycrystalline silicon rod to be used for a raw material for productionof single crystal silicon by an FZ method, wherein the polycrystallinesilicon rod is a polycrystalline silicon rod having an entire lengthgrown by a Siemens method of 500 mm or larger; and a side surface of thepolycrystalline silicon rod is cylindrically ground so that a differenceΔD₁ between a maximum value D_(max) and a minimum value D_(min) ofdiameters over the entire length becomes 3 mm or smaller.

Further the polycrystalline silicon rod according to the presentinvention is a polycrystalline silicon rod to be used as a raw materialfor production of single crystal silicon by an FZ method, wherein thepolycrystalline silicon rod has an entire length of 500 mm or larger;and the difference ΔD₂ between a maximum value D_(max) and a minimumvalue D_(min) of diameters of a nearly cylindrical virtual rotating bodyformed when the polycrystalline silicon rod is rotated about the centeraxis in the extension direction thereof is 6 mm or smaller.

In the case where the shape of a polycrystalline silicon rod right afterbeing obtained by a Siemens method does not satisfy the aboveconditions, as described above, the polycrystalline silicon rod comes tobe formed by cylindrically grinding the side surface of thepolycrystalline silicon rod.

That is, the method for processing a polycrystalline silicon rodaccording to the present invention is a method for processing apolycrystalline silicon rod to be used for a raw material for productionof single crystal silicon by an FZ method, wherein the polycrystallinesilicon rod is a polycrystalline silicon rod having an entire lengthgrown by a Siemens method of 500 mm or larger; and a side surface of thepolycrystalline silicon rod is cylindrically ground so that thedifference ΔD₂ between a maximum value D_(max) and a minimum valueD_(min) of diameters of a nearly cylindrical virtual rotating bodyformed when the polycrystalline silicon rod is rotated about the centeraxis in the extension direction thereof becomes 6 mm or smaller.

[Homogeneity in Crystal Characteristics]

As described above, any of the evaluation methods disclosed in PatentLiteratures 2 to 5 is a method using values of a crystal characteristic(or its distributions) in the growth direction (radial direction) of apolycrystalline silicon rod. However, the polycrystalline silicon rod iscylindrical; so no consideration of values of a crystal characteristic(or its distributions) in the extension direction (axial direction) isnot suitable for suppressing the loss of crystal habit lines in an FZoperation.

Further when single crystal silicon is produced by an FZ method, sincethe single crystal is grown by melting, zone melting and solidificationof a polycrystalline silicon rod while the polycrystalline silicon rodis clasped and rotated, also in the evaluation of values of a crystalcharacteristic (or its distributions) in the growth direction (radialdirection) of the polycrystalline silicon rod, their rotational symmetryis needed to be evaluated.

Here, since the polycrystalline silicon rod is cylindrical, the amountof polycrystals present at a growth distance d from its center comes tobe proportional to the square of d; and the amount of polycrystalsbecomes smaller on the nearer-center side, and that becomes larger onthe more outer side. The present inventors have had an idea that factorsaffecting the thermal stability, the thermal load, and the convection ofa melt in a melting interior, in an FZ process, are not simply values ofa crystal characteristic but evaluation values (values ofcharacteristic×amount of crystals) obtained by multiplying them by theamount of polycrystals contributing to the values of a crystalcharacteristic, and the idea has led to the present invention. That is,the present inventors have decided that novel evaluation values (valuesof a characteristic×amount of crystals) including the amount of crystalsgrown in the growth direction (radial direction) are to be defined andthe homogeneity in crystal characteristics in the growth direction(radial direction) is to be evaluated. Here, the important point in thepresent invention is such that since the amount of crystals is arelative amount of polycrystals at a specimen sampling position, theabove “amount of crystals” need not be an “absolute amount of crystals”,instead, may be a “relative amount of crystals”. In that sense, theabove “evaluation values” may be values of characteristics×“relativeamount of crystals”.

The evaluation of the homogeneity in crystal characteristics in theextension direction (axial direction) is similar, and comes to becarried out by using the above evaluation values (values of acharacteristic×relative amount of crystals).

The above-mentioned evaluation values (values of acharacteristic×relative amount of crystals) are, for example, thefollowing.

FIG. 1 is a view for interpreting one example of a sampling method ofspecimens provided for evaluation of values of a characteristic; and inthe example illustrated in the figure, a rod 11 of about 20 mm indiameter and about 750 mm in length is scooped out perpendicularly tothe axial direction of a polycrystalline silicon rod from a portion inthe vicinity of a bridge of the polycrystalline silicon rod 10 of about150 mm in diameter grown by deposition on a silicon core wire 1 by aSiemens method, that is, from a side-surface side of a site on a highestposition of the polycrystalline silicon rod 10 in a furnace, and 16sheets in total of specimen plates 12 of about 2 mm in thickness aresampled at 10-mm intervals from the rod 11. Then, for each of thesespecimen plates 12, Bragg reflection intensities from Miller indexplanes <111> and <220> are determined.

Similarly, a rod 11 of about 20 mm in diameter and about 750 mm inlength is scooped out perpendicularly to the axial direction of thepolycrystalline silicon rod from a portion in the vicinity of anelectrode of the polycrystalline silicon rod 10, that is, from aside-surface side of a site on a lowest position of the polycrystallinesilicon rod 10 in the furnace, and 16 sheets in total of specimen plates12 of about 2 mm in thickness are sampled at 10-mm intervals from therod 11. Then, for each of these specimen plates 12, Bragg reflectionintensities from Miller index planes <111> and <220> are determined.

In the present invention, the symmetry in the growth direction (radialdirection) of each site of the polycrystalline silicon rod is evaluatedby the product of a diffraction intensity value as a value of a crystalcharacteristic and a relative amount of crystals. Here, thedetermination of the “relative amount of crystals” to calculate theevaluation value involves first determining d² by squaring a distance d,from the center, of a site from which each specimen plate has beensampled, and determining Δd², which is a difference in the d² valuebetween specimen plates sampled adjacently.

Table 1 collectively shows sampling positions (distances from thecenter: d) of 16 sheets in total of specimen plates (SPL) sampled from aportion in the vicinity of the bridge (site positioned at the highestposition in the furnace), values d² obtained by squaring the d,differences Δd² in the d² value between adjacent specimen plates, Braggreflection intensities (I^(<111>) and I^(<220>)) from Miller indexplanes <111> and <220> being measurement values, and evaluation values(I^(<111>)×Δd² and I^(<220>)×Δd²) obtained by respectively multiplyingthe I^(<111>) and I^(<220>) by the differences Δd². Here, the averagevalue of the “rotational symmetries” is 88% for <111>, and 89% for<220>.

TABLE 1 Distance Measurement Evaluation Rotational from Center ValueValue Symmetry SPL. d I^(<111>) I^(<220>) I^(<111>) × I^(<220>) × <111><220> No. (cm) d² Δd² (cps) (cps) Δd² Δd² (%) (%) 1 7 49 13 9,161 9,337119,095 121,375 88.4 91.7 2 6 36 11 9,545 9,922 104,992 109,139 81.694.6 3 5 25 9 9,110 9,149 81,991 82,340 80.6 77.8 4 4 16 7 9,354 10,90965,481 76,362 99.5 99.2 5 3 9 5 8,097 10,919 40,484 54,595 90.5 97.8 6 24 3 8,587 13,686 25,762 41,057 88.9 83.2 7 1 1 1 8,489 13,310 8,40413,177 98.2 99.1 8 0.1 0 0.01 12,272 8,085 123 81 79.8 65.9 9 0.1 0 0.0110,025 11,409 100 114 10 1 1 1 8,335 13,189 8,252 13,057 11 2 4 3 9,59616,198 28,787 48,593 12 3 9 5 8,907 10,684 44,537 53,422 13 4 16 7 9,30910,818 65,164 75,726 14 5 25 9 7,500 7,324 67,501 65,913 15 6 36 117,934 9,399 87,275 103,387 16 7 49 13 8,154 8,594 105,998 111,718

Table 2 collectively shows sampling positions (d) of 16 sheets in totalof specimen plates (SPL) sampled from a portion in the vicinity of theelectrode (site positioned at the lowest position in the furnace),values d² obtained by squaring the d, differences Δd² in the d² valuebetween adjacent specimen plates, Bragg reflection intensities(I^(<111>) and I^(220>)) from Miller index planes <111> and <220> beingmeasurement values, and evaluation values (I^(<111>)×Δd² andI^(<220>)×Δd²) obtained by respectively multiplying the I^(<111>) andI^(<220>) by the differences Δd². Here, the average value of the“rotational symmetries” is 91% for <111>, and 84% for <220>.

TABLE 2 Distance Measurement Evaluation Rotational from Center ValueValue Symmetry SPL. d I^(<111>) I^(<220>) I^(<111>) × I^(<220>) × <111><220> No. (cm) d² Δd² (cps) (cps) Δd² Δd² (%) (%) 1 7 49 13 8,764 9,320113,938 121,154 96.9 90.8 2 6 36 11 8,728 8,753 96,013 96,280 97.2 92.53 5 25 9 8,642 8,783 77,774 79,043 87.2 95.9 4 4 16 7 8,985 7,486 62,89252,399 94.7 84.4 5 3 9 5 9,267 11,092 46,336 55,461 97.6 96.2 6 2 4 38,969 11,219 26,906 33,656 87.7 90.3 7 1 1 1 9,648 14,925 9,552 14,77585.9 55.8 8 0.1 0 0.01 9,664 6,258 97 63 78.2 62.8 9 0.1 0 0.01 12,0309,119 120 91 10 1 1 1 8,381 9,524 8,298 9,428 11 2 4 3 7,930 10,18423,789 30,552 12 3 9 5 9,495 10,676 47,474 53,382 13 4 16 7 9,471 8,75466,297 61,277 14 5 25 9 9,824 9,147 88,418 82,324 15 6 36 11 8,490 8,12293,392 89,345 16 7 49 13 8,500 8,500 110,503 110,505

FIG. 2 is a graphic diagram of the evaluation values obtained from these32 sheets of specimen plates.

The “rotational symmetry” in Table 1 and Table 2 is a value indicated in% according to the following procedure using a difference betweenevaluation values acquired from specimen plates positioned at axiallysymmetrical equal distances from the center axis. For example, in Table1, since a specimen plate 6 is positioned at a distance d of 2 cm fromthe center, a specimen plate positioned at an axially symmetrical equaldistance therefrom is a specimen 11; with respect to the evaluationvalue for the Miller index plane <111>, since that of the specimen 6 is25,762 and that of the specimen 11 is 28,787, the difference is 3,025. Avalue indicated in % by dividing the value 3,025 by 27,275 of theaverage of the above evaluation values becomes 11.1%. A value obtainedby subtracting 11.1% from 100% becomes 88.9%, and this value isdetermined to be a “rotational symmetry”.

Specimens positioned at different sampling positions in the heightdirection are also subjected to the similar calculation, and the degreeof the difference in the axial direction (height direction in a furnace)of the polycrystalline silicon rod is evaluated. For example, since thespecimen plate 6 shown in Table 1 is positioned at a distance d of 2 cmfrom the center, a specimen in Table 2 corresponding to this specimen isa specimen 6. With respect to the evaluation value for the Miller indexplane <111> of these specimens, since that of the specimen 6 in Table 1is 25,762 and that of the specimen in Table 2 is 26,906, the differenceis 1,144. A value indicated in % by dividing the value 1,144 by 26,334of the average of the above evaluation values becomes 4.3%. The 4.3% isdetermined to be a “difference in rotational symmetry” in the axialdirection.

Table 3 collectively shows “differences in rotational symmetry” in theaxial direction determined by the above-mentioned calculation. Here, theaverage value of the “differences in rotational symmetry” is 10% for<111>, and 17% for <220>.

TABLE 3 Difference in Rotational Symmetry SPL. Distance from Center(axial direction: %) No. d (cm) d² Δd² <111> <220> 1 7 49 13 4.4 0.2 2 636 11 8.9 12.5 3 5 25 9 5.3 4.1 4 4 16 7 4.0 37.2 5 3 9 5 13.5 1.6 6 2 43 4.3 19.8 7 1 1 1 12.8 11.4 8 0.1 0 0.01 23.8 25.5 9 0.1 0 0.01 18.222.3 10 1 1 1 0.5 32.3 11 2 4 3 19.0 45.6 12 3 9 5 6.4 0.1 13 4 16 7 1.721.1 14 5 25 9 26.8 22.1 15 6 36 11 6.8 14.6 16 7 49 13 4.2 1.1

In the present invention, a polycrystalline silicon rod having anabove-mentioned “rotational symmetry” of 40% or higher is employed as apolycrystalline silicon rod suitable as a raw material for production ofsingle crystal silicon by an FZ method. Further a polycrystallinesilicon rod having an above-mentioned “difference in rotationalsymmetry” of 40% or lower is employed as a polycrystalline silicon rodsuitable as a raw material for production of single crystal silicon byan FZ method.

The establishment of a criterion of the “difference in rotationalsymmetry” at 40% or lower is based on that when from relations betweenvalues of actual FZ, L % and resultant values of evaluation valuescalculated from values of characteristics, values of differences in theevaluation values when the obtained values of FZ, L % were 66% or higherwere determined, the values of differences were 40% or higher. Here, itwas judged that the number of n in the judgment of the establishment wassufficient and there was recognized a highly significant correlationagainst the criterion calculated from the correlation calculatedstatistically at a significant level of 1%.

Here, the “FZ, L %”, when the length of a polycrystalline silicon rod istaken to be 100, refers to a length indicated in %, of thepolycrystalline silicon rod, where crystal habit lines present in thecrystalline silicon obtained by crystal growth in one-time FZ operationare present in the complete state. Incomplete crystal habit lines ofcourse include lost crystal habit lines and also include disorderedones, bent ones and the like. Values of FZ, L % used in the presentdescription are all those obtained by a one-time FZ operation, and arenot those obtained by FZ operations in plural times. This is becausecarrying out of FZ operations in twice or more brings about reduction inproductivity, and the twice or more FZ operations is not included insubjects in the present invention.

The value of FZ, L % does not necessarily need to be 100%; but a highervalue thereof is better and the lower limit value thereof is, inconsideration of productivity, about 60 to 70% and a polycrystallinesilicon rod having the value lower than that cannot be used practically.

There are no special restrictions on what are the above-mentionedcrystal characteristics, but examples thereof may include values ofphysical properties such as crystal formation amount, crystalorientation, crystal grain diameter, thermal diffusivity and thermalconductivity, and the distributions of these values of physicalproperties may be employed as crystal characteristics.

The procedure of measuring a diffraction intensity from a specificMiller index plane by using an X-ray diffractometry may be the same asthat described in Patent Literatures 2 and 3.

Further the procedures of evaluating the crystal grain diameter and thecrystal grain diameter distribution may be the same as those describedin Patent Literature 4.

Further the procedures of evaluating the thermal diffusivity and thethermal conductivity may be the same as those described in PatentLiterature 5.

The “rotational symmetry” has usually a tendency of being lower in thevicinity of the electrode than in the vicinity of the bridge. Thistendency is similar even if the various conditions in thepolycrystalline silicon deposition are varied. Crystal sites positionedat low positions in the furnace, since being near the carbon-madeelectrode cooled in a reaction process, conceivably have a lower CVDtemperature than crystal sites positioned at high positions in thefurnace.

Since this phenomenon cannot be solved unless the fundamental spatialstructure arrangement and gas flow distribution of the Siemens methodare changed, if a polycrystalline silicon rod in a certain distance fromthe carbon electrode position is cut and discarded as a solving measure,the length of the silicon rod for FZ is resultantly made to beshortened. Here is a need to determine what degree of the distance is tobe secured, based on the homogeneity as an index in the height directionof crystals.

Although there can be thought of a method in which in order to securethe homogeneity in the axial direction, a reaction furnace is remodeledto be longish to elongate the length of a thermally flat portion, evenunder such a situation, the temperature reduction in the vicinity of thecarbon electrode cannot be avoided and heterogeneous portions ofcrystals come to be generated in a certain proportion.

It has been found that with respect to the dependency of crystalcharacteristics of the polycrystalline silicon rod on sites in theheight direction in the furnace, the homogeneity of crystals thusgradually decreases from high positions toward low positions. Therefore,in order to grasp the evaluation values of the polycrystalline siliconrod as a whole, there arises the need to evaluate specimens sampled fromboth end sites of the polycrystalline silicon rod.

When the evaluation of both the end sites is carried out, since themiddle region is expected to be in the range of the evaluation values ofthese two sites, there is no need to positively evaluate the otherregion than both the end sites. That is, the evaluation of two positionsin the height direction determines the evaluation values of thepolycrystalline silicon rod as a whole, and rough profiles in the rodcan be grasped. The present invention is, unlike simple measurementmethods of physical properties in the growth direction (radialdirection) as disclosed in Patent Literatures 2 to 5, an evaluationmethod of introducing novel evaluation values in which the amount ofcrystals in the radial direction is additionally taken intoconsideration, and further of also taking the (rotational) symmetry ofcrystals in the axial direction into consideration.

According to experiments carried out by the present inventors, whenpolycrystalline silicon rods having a “rotational symmetry” evaluated bythe above-mentioned means of 40% or higher were used as a raw materialfor production of single crystal silicon by an FZ method, single crystalingots having a proportion, of a length region where no loss of crystalhabit lines was recognized, of 66% were obtained. Any of suchpolycrystalline silicon rods had the above-mentioned “difference inrotational symmetry” of 40% or lower. That is, when polycrystallinesilicon rods having a “difference in rotational symmetry” of 40% orlower is used as a raw material for production of single crystal siliconby an FZ method, there are obtained single crystal ingots having aproportion, of a length region where no loss of crystal habit lines isrecognized, of 66%.

Further since the homogeneity of crystals is generally lower in thevicinity of the carbon electrode than in the vicinity of the bridge,when zone melting is initiated in single crystal silicon productionprocess by an FZ method, the initiation from a portion in the vicinityof the bridge more hardly brings about the loss of crystal habit lines.The reason therefor is conceivably that crystal habit lines have atendency of continuation in which the crystal habit lines are, even whenmany or few fluctuations occur after the crystal habit lines have oncebeen formed, zone melted as they are, and in the case where fluctuationsout of the acceptable range occur, the crystal habit lines are lost.

For these reasons, in the case where the zone melting is initiated froma portion in the vicinity of the carbon electrode, a possibility of lossof crystal habit lines in the course of the zone melting becomes high;thus, the case becomes more disadvantageous than the initiation from aportion in the vicinity of the bridge. Hence, it is desirable that thezone melting process for FZ be initiated from a portion in the vicinityof the bridge.

Table 4 comparatively shows values of FZ, L % in the case where zonemelting was initiated from a portion in the vicinity of a bridge, andvalues of FZ, L % in the case where zone melting was initiated from aportion in the vicinity of an electrode, for each of fourpolycrystalline silicon rods (a, b, c and d).

TABLE 4 Initiated from Initiated from Polycrystalline Bridge VicinityElectrode Vicinity Silicon Rod (FZ, L %) (FZ, L %) a 100 91 b 76 69 c 6640 d 2 0

The method for evaluating crystals according to the present inventioncan be summarized as follows. That is, the method is a method forevaluating crystals of a polycrystalline silicon rod to be used as a rawmaterial for production of single crystal silicon by an FZ method,wherein a plurality of specimen plates each having, as the principalplane, a cross-section perpendicular to a radial direction of thepolycrystalline silicon rod grown by a Siemens method are sampled atequal intervals in the radial direction; values of a crystalcharacteristic of the specimen plates are determined by measurements;and the homogeneity of the polycrystalline silicon rod is evaluated fromevaluation values obtained by multiplying the amount of crystals(relative amount of crystals) at sites where the specimen plates havebeen sampled by the values of crystal characteristics.

Further the means for evaluating the “rotational symmetry” employed inthe present invention can be summarized as follows. That is, the meanscomprises a step of determining the above evaluation values of twospecimens sampled from sites at symmetrical positions about the centeraxis of the polycrystalline silicon rod, calculating a rotationalsymmetry from the obtained two evaluation values A and B by thefollowing expression:100−(|A−B|/(A+B)/2)×100,and evaluating whether or not the average value in the axial directionof the rotational symmetries is 40% or higher.

Further the means for evaluating the “difference in rotational symmetry”employed in the present invention can be summarized as follows. That is,the means comprises a step of determining the above evaluation values oftwo specimens sampled from sites positioned at equal distances in theradial direction from the center axis of the polycrystalline silicon rodand on the same generating line in the case where the polycrystallinesilicon rod is approximated to a cylinder, calculating a difference inrotational symmetry from the obtained two evaluation values C and D bythe following expression:(|C−D|/(C+D)/2)×100,and evaluating whether or not the average value in the axial directionof the differences in rotational symmetry is 40% or lower.

Hereinafter, Experimental Examples (Examples and Comparative Examples)will be described.

Experimental Example 1

Experimental Example 1 is a study of the influence of the external shapeof a polycrystalline silicon rod on FZ, L %. The study results are shownin Table 5. Comparative Examples were polycrystalline silicon rods asthey were synthesized by a Siemens method; and Examples were thepolycrystalline silicon rods formed by cylindrically grinding the abovepolycrystalline silicon rods.

TABLE 5 Comparative Examples Examples Diameter Diameter FZ, (mm) ΔD₁ ΔD₂FZ, L % (mm) ΔD₁ ΔD₂ L % 153-158 5 9 18 153-158 3 6 74 130-134 4 7 24130-134 3 5 68

In all Examples, the difference ΔD₁ between a maximum value D_(max) anda minimum value D_(min) of diameters over the entire length was 3 mm orsmaller, and the difference ΔD₂ between a maximum value D_(max) and aminimum value D_(min) of diameters of a nearly cylindrical virtualrotating body formed at a time when the polycrystalline silicon rod wasrotated about the center axis in the extension direction was 6 mm orsmaller. By contrast, in all Comparative Examples, the ΔD₁ was largerthan 3 mm and also the ΔD₂ was larger than 6 mm.

Then, in all Examples, the FZ, L % exceeded 66, whereas in allComparative Examples, the FZ, L % was far below this.

Experimental Example 2

The results of Experimental Example 2 are shown in Tables 1 to 3.

As described already, in the present invention, a polycrystallinesilicon rod having a “rotational symmetry” of 40% or higher is adoptedas a polycrystalline silicon rod suitable as a raw material forproduction of single crystal silicon by an FZ method. A polycrystallinesilicon rod having the above “difference in rotational symmetry” of 40%or lower as a polycrystalline silicon rod suitable is adopted as a rawmaterial for production of single crystal silicon by an FZ method.

Experimental Example 3

In Experimental Example 3, for specimens sampled from the vicinity ofthe bridge of each of 5 polycrystalline silicon rods having differentFZ, L % (100, 76, 66, 52 and 2) and specimens sampled from the vicinityof the electrode thereof, the rotational symmetry (average value) andthe difference in rotational symmetry (average value) were investigated.In Examples here, there were employed Bragg reflection intensities fromthe Miller index planes <111> and <220> as crystal characteristics fordetermining evaluation values. The results are shown collectively inTable 6.

TABLE 6 Difference in Rotational Rotational Rotational Symmetry SymmetrySymmetry (bridge (electrode (axial vicinity: %) vicinity: %) direction:%) FZ, L % <111> <220> <111> <220> <111> <220> 100 88 89 91 84 10 17 7657 53 45 41 25 28 66 41 40 42 40 35 39 52 30 29 28 30 55 50 2 9 7 10 985 95

According to the results collectively shown in the table, the case wherethe average value in the axial direction of the rotational symmetrieswas 40% or higher, and the average value in the axial direction of thedifferences in rotational symmetry was 40% or lower had an FZ, L % of66% or higher.

Experimental Example 4

Also in Experimental Example 4, for specimens sampled from the vicinityof the bridge of each of 5 polycrystalline silicon rods having differentFZ, L % (100, 76, 66, 52 and 2) and specimens sampled from the vicinityof the electrode thereof, the rotational symmetry (average value) andthe difference in rotational symmetry (average value) were investigated.In Examples here, there were employed the orientations of the Millerindex planes <111> and <220> as crystal characteristics for determiningevaluation values; and specifically, there were employed areal ratios(%) of diffraction intensities from the Miller index planes <111> and<220>. The results are shown collectively in Table 7.

TABLE 7 Difference in Rotational Rotational Rotational Symmetry SymmetrySymmetry (bridge (electrode (axial vicinity: %) vicinity: %) direction:%) FZ, L % <111> <220> <111> <220> <111> <220> 100 64 63 74 78 27 31 7668 55 69 50 28 35 66 48 40 51 40 32 40 52 15 13 9 8 48 52 2 6 4 5 4 7996

Also in the results collectively shown in the table, the case where theaverage value in the axial direction of the rotational symmetries was40% or higher, and the average value in the axial direction of thedifferences in rotational symmetry was 40% or lower had an FZ, L % of66% or higher.

Experimental Example 5

Also in Experimental Example 5, for specimens sampled from the vicinityof the bridge of each of 5 polycrystalline silicon rods having differentFZ, L % (100, 76, 66, 52 and 2) and specimens sampled from the vicinityof the electrode thereof, the rotational symmetry (average value) andthe difference in rotational symmetry (average value) were investigated.In Examples here, there were employed average particle diameterdistributions measured by an EBSD method as crystal characteristics fordetermining evaluation values. The results are shown collectively inTable 8.

TABLE 8 Difference in Rotational Rotational Rotational Symmetry SymmetrySymmetry (bridge (electrode (axial FZ, L % vicinity: %) vicinity: %)direction: %) 100 70 60 25 76 55 50 30 66 45 40 38 52 33 28 52 2 4 4 96

According to the results collectively shown in the table, the case wherethe average value in the axial direction of the rotational symmetrieswas 40% or higher, and the average value in the axial direction of thedifferences in rotational symmetry was 40% or lower had an FZ, L % of66% or higher.

Experimental Example 6

Also in Experimental Example 6, for specimens sampled from the vicinityof the bridge of each of 5 polycrystalline silicon rods having differentFZ, L % (100, 76, 66, 52 and 2) and specimens sampled from the vicinityof the electrode thereof, the rotational symmetry (average value) andthe difference in rotational symmetry (average value) were investigated.In Examples here, there were employed the thermal diffusivities ascrystal characteristics for determining evaluation values. Here, sincethe thermal conductivity is a value obtained by multiplying the thermaldiffusivity by the density and the specific heat, values of both have nodifference. The results are shown collectively in Table 9.

TABLE 9 Difference in Rotational Rotational Rotational Symmetry SymmetrySymmetry (bridge (electrode (axial FZ, L % vicinity: %) vicinity: %)direction: %) 100 95 93 29 76 87 81 35 66 55 41 38 52 42 39 45 2 4 4 91

According to the results collectively shown in the table, the case wherethe average value in the axial direction of the rotational symmetrieswas 40% or higher, and the average value in the axial direction of thedifferences in rotational symmetry was 40% or lower had an FZ, L % of66% or higher.

Experimental Example 7

In Experimental Example 7, a pair (two wires) of silicon core wires wasassembled in a reverse U-shape through a bridge and polycrystallinesilicon was deposited; one polycrystalline silicon rod (two rods intotal) collected from each of both leg sections of the depositedpolycrystalline silicon was used; for the one rod (polycrystallinesilicon rod), an FZ process was initiated from a portion in the vicinityof the bridge and for the other one (polycrystalline silicon rod), theFZ process was initiated from a portion in the vicinity of theelectrode; and obtained values for FZ, L % of the single crystal siliconwere compared. Here, the polycrystalline silicon used in the experimentwas included ones grown by four different Siemens processes of A to D.The results are shown in Table 10.

TABLE 10 Initiated from Initiated from Polycrystalline Bridge VicinityElectrode Vicinity Silicon Portion Portion A 100 91 B 76 69 C 66 40 D 20

According to the results collectively shown in the table, even ifpolycrystalline silicon rods essentially of the same quality were used,the case where the FZ process was initiated from a portion in thevicinity of the bridge exhibited a higher value of FZ, L % than the casewhere the FZ process was initiated from a portion in the vicinity of theelectrode.

Therefore, in the case where when FZ single crystal silicon is producedby using a polycrystalline silicon rod as a raw material, there is used,as the polycrystalline silicon rod, a polycrystalline silicon rodcollected from a leg section of a reverse U-shape polycrystallinesilicon section obtained by assembling one pair (two wires) of siliconcore wires through a bridge in a reverse U-shape and depositingpolycrystalline silicon, when one end side of the polycrystallinesilicon rod is made to be a portion in the vicinity of the bridge in adepositing furnace, and the other end side is made to be a portion inthe vicinity of the electrode in the depositing furnace, it ispreferable that the floating zone melting step be initiated from theportion in the vicinity of the bridge of the polycrystalline siliconrod.

INDUSTRIAL APPLICABILITY

The present invention provides a technology for providing apolycrystalline silicon rod, excellent in the homogeneity in shape andthe homogeneity in crystal characteristics, suitable as a raw materialfor production of single crystal silicon by an FZ method.

REFERENCE SIGNS LIST

-   1 SILICON CORE WIRE-   10 POLYCRYSTALLINE SILICON ROD-   11 ROD-   12 SPECIMEN PLATE

The invention claimed is:
 1. A polycrystalline silicon rod to be used asa raw material for production of single crystal silicon by a float zone(FZ) melting method, wherein the polycrystalline silicon rod has anentire length of 500 mm or larger and an average diameter of 130 mm orlarger and 300 mm or smaller, a circularity of a plurality ofcross-sections perpendicular to a cylindrical shaft of thepolycrystalline silicon rod taken over the entire length of thepolycrystalline silicon rod is configured so that a first difference ΔD₁between a first maximum value and a first minimum value of diameters oftwo concentric circles of each of the plurality of cross-sections is 3mm or smaller over the entire length of the polycrystalline silicon rod,the first difference ΔD₁ being determined by measuring a difference(ΔD₁/2) between radii of the two concentric circles of each of theplurality of cross-sections, a second difference ΔD₂ between a secondmaximum value and a second minimum value of diameters of a nearlycylindrical virtual rotating body formed when the polycrystallinesilicon rod is rotated about a center axis in an extension directionthereof is 6 mm or smaller, an average rotational symmetry of thepolycrystalline silicon rod over the entire length of thepolycrystalline silicon rod is 40% or more, and the average rotationalsymmetry is calculated by: (i) sampling a plurality of specimen plateseach having, as a principal plane thereof, a cross-section perpendicularto a radial direction of the polycrystalline silicon rod, the pluralityof specimen plates being sampled at equal intervals in the radialdirection; (ii) determining evaluation values A, B of a pair of theplurality of specimen plates sampled from sites at symmetrical positionsabout a center axis of the polycrystalline silicon rod, wherein theevaluation value A is determined as a function of a crystalcharacteristic of a first specimen plate sampled from a site, positionedat a first distance from the center axis of the polycrystalline siliconrod, in the polycrystalline silicon rod, and the evaluation value B isdetermined as a function of the crystal characteristic of a secondspecimen plate sampled from a site, positioned at a second distance thatis symmetrically equal to the first distance with respect to the centeraxis of the polycrystalline silicon rod, in the polycrystalline siliconrod; (iii) calculating a rotational symmetry from the evaluation valuesA and B, which are related to homogeneity in the crystal characteristic,according to the equation: 100−[|A−B|/((A+B)/2)×100]; and (iv) repeatingsteps (i) to (iii) at different intervals along the entire length of thepolycrystalline silicon rod to obtain the average rotational symmetry,the crystal characteristic is one selected from the group consisting ofan amount of crystals formed, a crystal orientation, a crystal graindiameter, a thermal diffusivity, and a thermal conductivity.
 2. Apolycrystalline silicon rod to be used as a raw material for productionof single crystal silicon by a float zone (FZ) melting method, whereinthe polycrystalline silicon rod has an entire length of 500 mm or largerand an average diameter of 130 mm or larger and 300 mm or smaller, acircularity of a plurality of cross-sections perpendicular to acylindrical shaft of the polycrystalline silicon rod taken over theentire length of the polycrystalline silicon rod is configured so that afirst difference ΔD₁ between a first maximum value and a first minimumvalue of diameters of two concentric circles of each of the plurality ofcross-sections is 3 mm or smaller over the entire length of thepolycrystalline silicon rod, the first difference ΔD₁ being determinedby measuring a difference (ΔD₁/2) between radii of the two concentriccircles of each of the plurality of cross-sections, a second differenceΔD₂ between a second maximum value and a second minimum value ofdiameters of a nearly cylindrical virtual rotating body formed when thepolycrystalline silicon rod is rotated about a center axis in anextension direction thereof is 6 mm or smaller, an average difference inrotational symmetry of the polycrystalline silicon rod over the entirelength of the polycrystalline silicon rod is 40% or less, and theaverage difference in rotational symmetry is calculated by: (i)obtaining a first sampling rod from a first end of the polycrystallinesilicon rod, and a second sampling rod from a second end of thepolycrystalline silicon rod opposite to the first end in an axialdirection, (ii) sampling a plurality of specimen plates from each of thefirst and second sampling rods, each of the plurality of specimen plateshaving, as a principal plane thereof, a cross-section perpendicular toan axial direction of each of the first and second sampling rods, theplurality of specimen plates being sampled at equal intervals in theaxial direction; (iii) determining evaluation values C, D, wherein theevaluation value C is determined as a function of a crystalcharacteristic of a third specimen plate sampled from a site, positionedat a third distance from a center axis of the first sampling rod, in thepolycrystalline silicon rod, and the evaluation value D is determined asa function the crystal characteristic of a fourth specimen plate sampledfrom a site, positioned at a fourth distance that is equal to the thirddistance from a center of the second sampling rod, in thepolycrystalline silicon rod; (iv) calculating a difference in rotationalsymmetry from the evaluation values C and D, which are related tohomogeneity in the crystal characteristic, according to the equation:[|C−D|/((C+D)/2)/2]×100; and (v) repeating steps (i) to (iv) atdifferent intervals along the entire length of the polycrystallinesilicon rod to obtain the average difference in rotational symmetry, thecrystal characteristic is one selected from the group consisting of anamount of crystals formed, a crystal orientation, a crystal graindiameter, a thermal diffusivity, and a thermal conductivity.
 3. Thepolycrystalline silicon rod according to claim 1, wherein the crystalcharacteristic is the crystal orientation, and wherein the crystalcharacteristic is obtained by Bragg reflection intensity from Millerindex plane <111> or Miller index plane <220>.
 4. The polycrystallinesilicon rod according to claim 1, wherein the evaluation values A and Bare each calculated as a function of the crystal characteristic and adifference between the first and second distances, respectively, and adistance of an adjacent specimen plate from the center axis of thepolycrystalline silicon rod.
 5. The polycrystalline silicon rodaccording to claim 2, wherein the crystal characteristic is the crystalorientation, and wherein the crystal characteristic is obtained by Braggreflection intensity from Miller index plane <111> or Miller index plane<220>.
 6. The polycrystalline silicon rod according to claim 2, whereinthe evaluation values C and D are each calculated as a function of thecrystal characteristic and a difference between the third and fourthdistances, respectively, and a distance of an adjacent specimen platefrom the center axis of the first and second sampling rods,respectively.